Summary

Just a small typo :)

Docs said the default package static dir is static, but the code uses statics:
https://github.com/Kludex/starlette/blob/de970d7b3facb853eb7ad077decbf3d94f2aab6c/starlette/staticfiles.py#L75

...Unless I misunderstood something.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.